is_wroff_extra
connp->conn_wroff = len + is->is_wroff_extra;
wroff = connp->conn_ht_iphc_allocated + is->is_wroff_extra;
flowinfo, 0, data_mp, data_len, is->is_wroff_extra, &cksum, errorp);
mp1 = allocb(copylen + is->is_wroff_extra, BPRI_MED);